This invention relates to a thermal imaging medium for the recordation of information. More particularly, it relates to an imaging medium especially adapted to the provision of thermally actuated images having improved handling, durability and abrasion-resistance characteristics.
The provision of images from media which rely upon the generation of heat patterns has been well known. Thermally imageable media are particularly advantageous inasmuch as they can be imaged without certain of the requirements attending the use of silver halide based media, such as darkroom processing and protection against ambient light. Moreover, the use of thermal imaging materials avoids the requirements of handling and disposing of silver-containing and other processing streams or effluent materials typically associated with the processing of silver halide based imaging materials.
Various methods and systems for preparing thermally generated symbols, patterns or other images have been reported. Examples of these can be found in U.S. Pat. No. 2,616,961 (issued Nov. 4, 1952 to J. Groak); in U.S. Pat. No. 3,257,942 (issued Jun. 28, 1966 to W. Ritzerfeld, et al.) in U.S. Pat. No. 3,396,401 (issued Aug. 6, 1968 to K. K. Nonomura); in U.S. Pat. No. 3,592,644 (issued Jul. 13, 1971 to M. N. Vrancken, et al.); in U.S. Pat. No. 3,632,376 (issued Jan. 4, 1972 to D. A. Newman); in U.S. Pat. No. 3,924,041 (issued Dec. 2, 1975 to M. Miyayama, et al.); in U.S. Pat. No. 4,123,578 (issued Oct. 31, 1978 to K. J. Perrington, et al.); in U.S. Pat. No. 4,157,412 (issued Jun. 5, 1979 to K. S. Deneau); in Great Britain Patent Specification 1,156,996 (published Jul. 2, 1969 by Pitney-Bowes, Inc.); and in International Patent Application No. PCT/US87/03249 of M. R. Etzel (published Jun. 16, 1988, as International Publication Number WO 88/04237).
In the aforementioned International Application, there is disclosed a thermal imaging medium which includes a layer of porous or particulate image-forming material and which is especially adapted to the provision of high resolution images by subjecting the medium to brief exposure to intense image-forming radiation. According to a preferred embodiment, the image-forming material (preferably, a layer of carbon black) is coated over the heat-activatable image-forming surface of a first sheet-like or web material and is covered with a second laminated sheet-like element, such that, the image-forming substance is confined between the sheet-like elements of a thermal imaging laminar medium. Upon exposure of the medium (for example, by laser scanning) and on separation of the sheet-like elements, a pair of images is obtained.
A first image comprises exposed portions of image-forming substance more firmly attached to the first sheet element by heat activation of the heat-activatable image-forming surface. A second image comprises non-exposed portions of the image-forming substance carried or transferred to the second sheet element.
The respective images obtained by separating the sheets of an exposed thermal imaging medium having an image-forming substance confined therebetween may exhibit substantially different characteristics. Apart from the imagewise complementary nature of these images and the relation that each may bear as a "positive" or "negative" of an original, the respective images may differ in character. Differences may depend upon the properties of the image-forming substance, on the presence of additional layer(s) in the medium, and upon the manner in which such layers fail adhesively or cohesively upon separation of the sheets. Either of the pair of images may, for reasons of informational content, aesthetics or otherwise, be desirably considered the principal image. The principal image may, however, depending upon the aforementioned properties and modes of failure, exhibit decidedly inferior properties, such as poorer handling characteristics, durability and abrasion resistance, as compared with the complementary image of secondary importance.
In the production of thermal images from media of the type described in the aforementioned International Application, it will oftentimes be preferred, in the case of high density images, that the principal image be that which is formed by transfer of non-exposed regions of coated image-forming substance to a sheet separated from an imaged medium. It will be recognized that an alternative is to form a high density image on the opposed sheet by firmly attaching the image-forming substance in areas of exposure. This is the case because the medium provides complementary images and the desired high density image can be formed on either sheet by addressing the thermally actuatable medium according to which sheet shall bear the high density image. This alternative to the formation of a high density image is, however, disadvantageous since the areas of high density are created in areas of exposure (by activation of a heat-activatable image-forming zone or layer) and large areas of exposure require correspondingly large areas of laser actuation and energy utilization and highly accurate laser scanning and tracking. Errors in tracking will result in discontinuities (whiteness or voids) by failure to attach minute regions of image-forming substance and by their removal to the opposed sheet upon separation of the sheets. Owing to the psychophysical nature of human vision, minute regions of lightness (voids) against an expansive darkness tend to be noticeable.
It will, thus, be preferred that a high density image be the result of the transfer in non-exposed regions of coated and continuous regions of image-forming material (with minimal or no discontinuities or coverage voids), rather than the result of firm connection of high density regions of imaging material by laser-actuated operation of the heat-activatable image-forming surface, where tracking errors increase the possibility of creating noticeable areas of discontinuity (whiteness) against the expansive high density region. Images comprising image-forming material transferred in non-exposed regions tend, however, to exhibit less durability and resistance to scratching than the complementary image. Inasmuch as such images may be preferred for the value of their informational content, and are especially desired for medical diagnostic purposes, there is an interest in providing a thermal imaging medium capable of providing such images with improved handling, durability and abrasion-resistance characteristics.